1. Field of the Invention (Technical Field)
The invention relates to soldering apparatuses and processes, in particular manufacturing uniform solder balls and soldering interconnects to a ball grid array (BGA) package.
2. Background Art
Package designs for microelectronic devices have moved from through-hole to surface mount technology in order to increase the useable printed wiring board area available by utilizing both sides of the boards. The traditional geometry for surface mount devices is peripheral arrays, where the leads are on the edges of the device. As the technology drives towards high input/output (I/O) count (increasing number of leads) and smaller packages with finer pitch (less distance between peripheral leads), limitations on peripheral surface mount devices arise. It is the opinion of experts in this field that the limit of manufacturability of peripherally leaded devices lies at a pitch of approximately 0.3 mm. The leads on these fine pitch devices are fragile and can be easily bent. At a 0.3 mm pitch, it becomes difficult to deliver solder paste by stencil printing or plating processes to leads spaced this closely. If there is too much solder mass, bridging between leads can occur. If not enough solder is applied, mechanical and electrical continuity can be lost.
A solution to the fine pitch issue is to shift the leads from the periphery of the device to the area under the device. This scheme is called areal array packaging and is exemplified by the ball grid array (BGA) package. The following example demonstrates the advantage of areal array versus peripheral, with regard to pitch. A 640 lead package (with a 50.8 mm side length) comprises a pitch on a peripheral device of 0.31 mm, while that of a BGA package has a pitch of 1.95 mm.
The current practice to join BGA packages to printed wiring boards involves a hierarchy of solder alloy compositions. A high melting temperature ball (e.g., 90 Pb--10 Sn that melts at 300.degree. C.) is typically used for a standoff. These balls are made by either a spray forming operation or by cutting wire to specific lengths then reflowing them in flux. Both techniques result in a range of solder ball size that requires a sieving operation to produce solder balls of uniform dimensions. The process to create the balls represents a significant fraction of the overall BGA package manufacturing cost and it is estimated that solder balls alone make up 13% of this cost. A lower melting temperature solder (e.g. 63 Sn--37 Pb near eutectic solder) paste is used to join the balls to the package substrate and board. One method to attach the high melting temperature balls to the substrate involves loading a graphite fixture, or boat, that is drilled with holes of the same diameter as the balls, in the desired areal array pattern. The package substrate is patterned with the lower melting temperature solder paste using either a dispensing method, screen printing, or stenciling the paste. The substrate and fixture are then placed together so that the balls make intimate contact with the solder paste. Another method to attach the balls is to use a chuck fitted with a machined template that has multiple apertures through which a vacuum attracts the solder balls. With some robotic vibration all holes are eventually filled and extra balls are shaken loose. The balls are then fluxed and placed in unison onto the substrate that has been deposited with solder paste. After the ball placement operation, the assembly goes through a reflow furnace that melts the solder paste and joins the paste to the balls and substrate. The fixture is then carefully removed so as to not damage the joined balls. The substrate is then cleaned to remove flux residue using a solvent and distilled water rinse. Final assembly involves aligning the substrate/ball composite to the board, providing lower melting temperature solder paste on the lands and reflowing and subsequent cleaning of the joints.
Another option to manufacture BGA solder joints is by use of monolithic ball metallurgy techniques where only lower melting temperature solder is utilized. Using this technique, a controlled volume of solder paste is dispensed, or screen printed onto the metallized pads on the boards. The BGA devices have the lower melting temperature solder balls reflowed onto the contacts and the two parts are aligned and reflowed. A variation of this scheme is to not use paste on the boards but just enough flux to break down surface oxides and then place solder balls on the fluxed substrate, (as discussed above using a fixture) then reflow the package to the board. The problems with this methodology are concerns over the formation of the solder balls from the paste or the introduction of voids into the joints from the volatiles in the flux.
U.S. Pat. No. 5,229,016 to Hayes, et al., entitled Method and Apparatus for Dispensing Spherical Shaped Quantities of Liquid Solder, U.S. Pat. No. 5,377,961 to Smith, et al., entitled Electrodynamic Pump for Dispensing Molten Solder and U.S. Pat. No. 4,981,249 to Kawashima, et al., entitled Automatic Jet Soldering Apparatus, are all nozzled devices for dispensing one droplet or single ball at a time. Therefore, to use these devices, either the nozzle must be moved to the desired location or the circuit board or other surface to be soldered must be moved to the location of the nozzle. These devices do not teach or disclose a multi-apertured device for simultaneously dispensing solder balls.
U.S. Pat. No. 5,356,658 to Hartz, et al., entitled Flexible High Speed Liquid Dispenser is a multi-apertured device. However, the multi-apertured plate is dipped into the liquid, thereby filling the apertures with the liquid and then placed over the surface to be dispensed, and air blown over the apertures causing the liquid in the apertures to extrude and cover the surface. This device will not work with solder because there is no means to keep the liquid at a certain temperature. Additionally, this is not a jetting device but teaches an extrusion process. In a jetting event a drop or stream of liquid is emitted from an aperture with sufficient kinetic energy to launch said liquid into flight. No mating surface need be in contact with the fluid during the jetting event. In most applications of jetting, a mating surface is used, but it can be placed a signficant distance from the jetting aperture. In an extrusion process the liquid emerges from the aperture with relatively low kinetic energy and is immediately contacted by the mating surface so that the liquid is never in free flight. An example of the extrusion process is silk screen printing. Finally, there are no controls to keep the dispensed liquid uniform in size in this prior art.